Well as dumb as it sounds, one reason to allow hyphens instead of requiring underscores is that the former takes one keystroke to type, whereas the latter takes two. This is kind of nice when dealing with longer function names.

It's also worth noting that pyret requires spaces between all math operators, not just subtraction. In other words, 3+4 is not valid; you would need to type 3 + 4. So at least it's internally consistent in that sense.